Hydraulic brake systems for automotive applications typically include a driver-actuated brake pressure generator, such as a booster-aided tandem master cylinder. The pressure generator connects to a reservoir and supplies pressurized brake fluid to a pair of brake circuits. Each brake circuit connects to two wheel brakes.
To provide an anti-lock function with the hydraulic brake system, each wheel brake circuit includes a normally open electronically operated inlet valve. The inlet valve controls the flow of pressurized hydraulic brake fluid to the wheel brake. A pressure relief line includes a normally closed electronically operated outlet valve and leads from each wheel brake to a low-pressure accumulator. The low-pressure accumulator receives brake fluid released during a pressure decrease phase when the outlet valve is opened. A suction line leads from the low-pressure accumulator to a suction side of a motor driven pump.
A pressure side of the pump is connected to a brake line between the inlet valve and the pressure generator. Separating valves are located in respective brake lines of each circuit upstream of the location at which the pressure relief line connects to the brake line. The separating valves isolate the brake lines from the pressure generator during electronically controlled brake operation.
During an active brake intervention event, when the driver has not activated the pressure generator, the pump is provided with the necessary brake fluid from the reservoir. To this end, an additional supply line typically connects the suction side of the pump with the reservoir or with the brake line that connects the pressure generator and the separating valve.
The pump is a self-priming pump that conveys brake fluid without a priming pressure at its suction side. The pump creates a vacuum at its suction side. A pressure retention valve is arranged in the suction line between the low-pressure accumulator and the location where the supply line connects to the suction line. The pressure retention valve prevents the pump from creating a vacuum in the wheel brake cylinders and the low-pressure accumulator. Generating a vacuum in the wheel brakes while braking on ice or other low-friction surfaces would lead to an increased volume of hydraulic brake fluid consumption and accordingly an increased brake pedal travel before the wheel brakes engage. This is an undesirable characteristic in such operating conditions.
The pressure retention valve is typically a check valve that is biased with a spring. The valve requires a pressure difference of approximately 1 bar to open the pressure retention valve and to permit the flow of brake fluid from the low-pressure accumulator to the pump. Due to variances in pressure retention valves, the nominal bias of 1 bar corresponds to an actual bias in the range of approximately 0.7-1.1 bar.
While the vehicle is operating normally on high-friction surfaces, it is desirable that the wheel brake friction elements are retracted to reduce drag and wear. However, the pressure retention valve, while useful in low-friction conditions, prevents the pump from fully retracting the wheel brake friction elements in such operating conditions.